1. Field of the Invention
This invention relates to electrodeless high intensity discharge lamps for general illumination applications. More particularly, this invention relates to a luminaire including novel geometric structures for the excitation coil of electrodeless high intensity discharge lamps wherein the excitation coil and capacitor plates are placed in close proximity to the lamp envelope with minimum obstruction to escape of useful light and which provide for efficient removal of heat from the coil without water cooling.
2. Technological Background
Electrodeless discharge lamps have been proposed whereby lamps operate using a solenoidal electric field created by an excitation coil. The electric field is substantially circular and closes back on itself. The discharge takes place along the electrical field path and, since it is a closed path, there is no need for electrodes. Such devices are described in U.S. Pat. Nos. 3,500,118 and, 4,180,763, both of which have issued to J. M. Anderson, one of the inventors herein, and both of which are assigned to the assignee of the present invention.
The invention herein relates to a class of electrodeless discharge lamps referred to as high intensity discharge (H.I.D.) lamps such as described for example in my prior U.S. Pat. No. 4,180,763 which is assigned to the assignee of the present invention. The subject matter of U.S. Pat. No. 4,180,763 is hereby incorporated by reference.
High intensity discharge lamps are distinguishable from ordinary fluorescent lamps in that H.I.D. lamps typically operate at a temperature of approximately 700.degree. C. or more and at a vapor pressure of between approximately 200 Torr and approximately one atmosphere. On the other hand, a low intensity discharge lamp, such as that described in U.S. Pat. No. 3,500,118, operates at a temperature of approximately 40.degree. C. and a vapor pressure of approximately 7 microns of mercury. High intensity discharge lamps typically consume a much greater amount of power and yield a correspondingly greater amount of optical output. The amount of power consumed depends in a positive way on the voltage drop along the discharge path.
For a high intensity discharge lamp, there is a typical electric field strength of approximately 10 volts per centimeter. For a more standard, low intensity discharge, the electric field strength is approximately 1 volt per centimeter. The higher power rating requires good coupling between the magnetic field and the electric field driven discharge. In addition, the higher power creates an increased heat energy output which requires a greater amount of heat dissipation and cooling capability than in low intensity discharge lamps. At the same time, improvements made in the coupling and heat sinking must not appreciably interfere with the visible light output of the lamp.
Another electrodeless plasma arc lamp employed as a high intensity light source utilizes an ionizable medium contained in an envelope and a radio frequency (RF) excitation coil immediately surrounding the envelope. This type of device suffers from the drawback that the excitation coil and other elements of the device that surround the envelope tend to impede the escape of light created within the arc envelope, thus decreasing the light emitting efficiency of the lamp.
Other devices propose to couple the input impedance of the excitation coil with the output impedance of a termination fixture, thus matching the impedance of the coil with that of the termination fixture. These devices provide a luminaire structure having long leads to the excitation coil and capacitors connected to the leads for creating an input impedance match. However, because of the parasitic impedance of the lead wires and the losses in the lead wires, the capacitors fail to provide efficient impedance matching.
Electrodeless high intensity discharge lamps suffer from a number of additional problems that must be dealt with. As previously noted, such lamps normally operate at very high temperatures, usually in a range around 700.degree. C. Moreover, the excitation coil usually operates at high temperatures which can range up to as much as 400.degree. C. These temperatures are created by resistive loss due to the large coil electric currents that develop the necessary arc voltage gradient which drives the device. A major practical consideration in the design of such lamps is the removal of excess heat from the coil during operation to limit losses in the coil, to preserve protective coatings and to regard corrosion of the coil material.
Prior art devices have relied on artificial cooling, such as flowing water, to provide for heat removal from the coil. Such cooling means make these devices unwieldy and the bulk associated with the cooling apparatus tends to obstruct the light and other radiation exiting the luminaire. Although water cooling is a viable method of cooling electrodeless H.I.D. lamps, it is impractical for general commercial uses of such lamps. A need exists for a practical method of cooling the excitation coils during the lamp operation.
Another practical consideration in the development of electrodeless H.I.D. lamps is the increased RF power loss in the metal wires of the excitation coil due to the skin effect, especially at the high frequencies in which the coil usually operates. One attempt at a solution to this problem is to design the coil such that it has very low resistance at the frequencies of operation.
In order to reduce the losses in the matching network and the ballast, resonating capacitors are needed directly across the coil. The high currents are thus confined between the resonating capacitors and the coil, thus allowing use of available low cost components in the matching network and the ballast. It has been found, moreover, that in order for the impedance matching to have greatest effectiveness, and in order to reduce the power loss in the connecting wires, the capacitors providing impedance matching should be in as close proximity to the excitation coil as possible. However, placement of the capacitors near the arc envelope tends to obstruct the light generated by the device, thus decreasing the efficiency of the lamp.